The present invention relates very generally to adjustable frequency motor control, and pertains, more particularly, to a pulse width modulation AC motor control that provides substantial improvement in operating parameters characterized in particular by cogless rotation even at low operating speeds.
In AC motor control, the AC motor control circuitry typically comprises an input rectifier and filter, a three-phase power inverter, and associated control circuitry for controlling the output voltage amplitude and frequency of the power inverter. In a pulse width modulated inverter, each output leg of the inverter is switched between high and low input potentials at a frequency which is much faster than the desired output frequency.
The output voltage of a leg of the inverter, referred to herein as the phase voltage, is usually referenced to the negative input potential. If a phase voltage is averaged over one of the high frequency switching cycles, the average value is proportional to the duty cycle, or the amount of time the output leg is switched to the positive input relative to the total cycle time. Thus, if the duty cycle is varied, in a periodic nature about some nominal duty cycle, the phase voltage has a proportional AC component oscillating about a nominal DC voltage. Increasing the modulation or the magnitude or the change in the duty cycle, increases the amplitude of the AC component of the phase voltage.
In the case of AC motor control, the motor is generally connected to three inverter output phases whose AC components are 120.degree. out of phase with each other. In such a connection, the phase-to-phase voltage applied to the load is the difference of the individual phase voltages. Thus, if the DC components of each phase are equal, the phase-to-phase voltage has only an AC component which is the vector difference of the AC components of the phases involved.
In the case wherein the DC components of two phase voltages are not equal, then a DC component is present in a resultant phase-to-phase voltage. With AC motors or other inductive loads, a DC voltage of even a few volts gives rise to significant DC current flows due to extremely low impedance of the devices at or near zero frequency.
Accordingly, when driving an inductive load such as a motor, it is desired that the high switching frequency remain high even at low output modulation frequencies. This is to allow the inductive nature of the load to limit the currents, due to high frequency switching, to a value which is small compared to the current due to the modulation amplitude. In this way the inductance of the motor then presents adequate impedance to maintain the peak currents small compared to the average current.
A common means for generating the appropriate switching command signals for the inverter is carrier modulation. With this technique, it is typical to synthesize three sine waves having the same frequency, amplitude and phase relationship desired of the output. These waveforms are compared with a common carrier waveform. The carrier is typically a sawtooth or triangular waveform at the desired switching frequency of the inverter. Any time that the modulation sine wave for a phase is larger than the carrier, the output leg of the inverter is switched to the high DC potential. Otherwise, the output is switched to the low DC potential. Thus, carrier modulation provides the appropriate duty cycle modulation of the switching cycle of the inverters.
In a simple implementation of carrier modulation, the carrier frequency is asynchronous, or bears no direct relationship with the modulation frequency. An advantage at low modulation frequencies with this simple technique is that the switching frequency of the inverter is maintained so that the peak currents due to carrier frequency switching are small compared to the fundamental current at the modulation or output frequency. The disadvantage of the asynchronous carrier technique is that the relationship of the carrier to each of the modulation waveforms is slightly different within a modulation cycle and will vary with a given modulation waveform from cycle-to-cycle. Such variations give rise to DC and low frequency AC current flows which have undesirable effects on the rotation of the motor.
A more complex technique that is presently employed is to synchronize the carrier frequency to a triplen multiple of the modulation frequency. The synchronous carrier approach eliminates beating between the carrier frequency modulation frequency present in the more simplified approach. However, in order to maintain the transistor switching frequencies within reasonable limits, the carrier frequency requires frequent changing to new multiples of the modulation frequency. Such changes are complicated to implement and not generally completely smooth.